Electrically-variable transmissions typically have an input member connected to an engine and one or two motor/generators connected to different members of planetary gear assemblies to allow one or more electrically-variable modes of operation, fixed speed ratio modes, and an electric-only (battery powered) mode. Electrically-variable transmissions may improve vehicle fuel economy in a variety of ways.
For instance, the motor generators are able to capture vehicle kinetic energy during braking, which is used by one of the motors acting as a generator to allow the engine to be turned off at idle, during periods of deceleration and braking, and during periods of low speed or light load operation to eliminate efficiency losses due to engine drag. Transient demand for engine torque or power is supplemented by the motor/generators during operation in engine-on, electrically-variable modes. The motor/generators keep the engine off longer, supplement engine torque or power and/or operate at a lower engine speed, or supplement accessory power supplies. Additionally, the motor/generators are very efficient in accessory power generation and electric power from the battery serves as an available torque reserve allowing operation at a relatively low transmission numerical speed ratio.
To extend the electrically variable modes of operation the motor/generator sizes are typically increased to provide additional power and speed to the transmission at peak demands.